Greetings!
I guess I came in late to this but here goes...
First, glad to know that you have at least AM and LSB, more than likely they had used it considerably for that...however if USB is "dead" - the best advice is to make sure the mode switch does work.
When other modes are ok, and one side failed. My experience has taught me to fix th simple stuff then onto more complex...
There are comparable radios - try this link...
http://www.cbtricks.com/radios/realistic/trc_465/index.htm
First things first, if the mode selector is not at fault - are you checking Solder joints for cracks?
What caused the failure - dropped or mishandled - extracted from a car wreck?
Possible broken wiring?
Broken wiring yes, the things got a ribbon cable from the front panel to the board.
So see if the mode switch is at fault you can probe the ribbon cable on the main PCB for voltages..and find which set of wires in the ribbon are bad. IF not voltages a simple continuity check from the mode switch back or the ribbon itself can prove good or bad. The voltage thingy is easier for quick needle swing, good, then onto the next one...
IF that test shows good, then more the likely the next culprit is the diode steering for power routing in the modes.
This is where you need to re-check the modes that do work for they can show you some insight as to why one is working.
Open the case, use board ground not case ground for your negative lead.
Power up the radio go to AM mode, find the mirror (jumper board) assembly that cover -shorts - three clips, this is the jumper that from the AM regulator located by the power connector UNDERNEATH it. More than likely this is ok, but I need you to do a voltage check on the mirrors assembly AS IT IS -DONT REMOVE IT FOR NOW... place a dummy load on the rear antenna connector to prevent accidents...
Take the + lead and check in AM mode the mirror assembly, what is your voltage - don't need to whistle or jump around just you DO NOT NEED TO key dead carrier into that dummy load.
Reading?
Do it again but this time in the other SSB modes...and YOU DO NOT NEED TO KEY...
Reading?
IF you get 4~5 Volts on AM you should - you say that mode works.
In USB? get Power supply 12~13.8 or whatever you're using for power feed - will be present here...
LSB - you should get the same thing...
Ok, let's check our readings...
AM - 5 Volts? Good ok move on...
LSB - 12 Volts? Ok, ok move on...
USB - 12 Volts? Ok, ok move on...
The above test is for MODE and if one of the poles is healthy - and if readings are like the above is what YOU get, that test passes...
IF YOU DONT - the MODE SWITCH may still be OK...!
You may have a Diode blown instead - so it pays to understand which of the symptoms are actually there - the more tests we do, the better choices on where to go to fix the problem...
Does the USB Mode make the TX light turn on? Yes, Ok, that test passes...
DOES USB MODE even transmit anything? As a signal into another radio nearby but you just don't have a receive?
Then we have isolated at least what can go wrong and what went wrong...
Ok, scenario time...
USB DOES NOT MAKE THE TX LIGHT WORK - means POWER FAILURE and PLL will not lock. Diode failed...or Cascaded failure of diode routing.
USB DOES MAKE THE TX LIGHT UP - MEANS that one side of the mixing is gone so you just don't have conversion - so more than likely 1/2 the radio functions. Diode failed...
But I'll stop here, report back what you find - when you can!
Regards!
:+> Andy <+: